# Introduction to Lena¶

In our data analysis we often face changing data or algorithms. For example, we may want to see how our analysis works for another dataset or for a specific subset of the data. We may also want to use different algorithms and compare their results.

To handle this gracefully, we must be able to easily change or extend our code at any specified point. The idea of Lena is to split our code into small independent blocks, which are later composed together. The tutorial will show us how to do that and what implications this idea will have for our code.

## The three ideas behind Lena¶

### 1. Sequences and elements¶

The basic idea of Lena is to join our computations into sequences. Sequences consist of elements.

The simplest Lena program may be the following. We use a sequence with one element, an anonymous function, which is created in Python by lambda keyword:

>>> from lena.core import Sequence
>>> s = Sequence(
...     lambda i: pow(-1, i) * (2 * i + 1),
... )
>>> results = s.run([0, 1, 2, 3])
>>> for res in results:
...     print(res)
1 -3 5 -7


The first line imports a Lena class Sequence. A Sequence can be initialized from several elements. To make the Sequence do the actual work, we use its method run. Run’s argument is an iterable (in this case a list of four numbers).

To obtain all results, we iterate them in the cycle for.

Let us move to a more complex example. It is often convenient not to pass any data to a function, which gets it somewhere else itself. In this case use a sequence Source:

from lena.core import Sequence, Source
from lena.flow import CountFrom, Slice

s = Sequence(
lambda i: pow(-1, i) * (2 * i + 1),
)
spi = Source(
CountFrom(0),
s,
Slice(10**6),
lambda x: 4./x,
Sum(),
)
results = list(spi())
# [3.1415916535897743]


The first element in Source must have a __call__ special method, which accepts no arguments and generates values itself. These values are propagated by the sequence: each following element receives as input the results of the previous element, and the sequence call gives the results of the last element.

A CountFrom is an element, which produces an infinite series of numbers. Elements must be functions or objects, but not classes . We pass the starting number to CountFrom during its initialization (in this case zero). The initialization arguments of CountFrom are start (by default zero) and step (by default one).

The following elements of a Source (if present) must be callables or objects with a method called run. They can form a simple Sequence themselves.

Sequences can be joined together. In our example, we use our previously defined sequence s as the second element of Source. There would be no difference if we used the lambda from s instead of s.

A Sequence can be placed before, after or inside another Sequence. A Sequence can’t be placed before a Source, because it doesn’t accept any incoming flow.

Note

If we try to instantiate a Sequence with a Source in the middle, the initialization will instantly fail and throw a LenaTypeError (a subtype of Python’s TypeError).

All Lena exceptions are subclassed from LenaException. They are raised as early as possible (not after a long analysis was fulfilled and discarded).

Since we can’t use an infinite series in practice, we must stop it at some point. We take the first million of its items using a Slice element. Slice and CountFrom are similar to islice and count functions from Python’s standard library module itertools. Slice can also be initialized with start, stop[, step] arguments, which allow to skip some initial or final subset of data (defined by its index), or take each step-th item (if the step is two, use all even indices from zero).

We apply a further transformation of data with a lambda, and sum the resulting values.

Finally, we materialize the results in a list, and obtain a rough approximation of pi.

### 2. Lazy evaluation¶

Let us look at the last element of the previous sequence. Its class has a method run, which accepts the incoming flow:

class Sum():
def run(self, flow):
s = 0
for val in flow:
s += val
yield s


Note that we give the final number not with return, but with yield. Yield is a Python keyword, which turns a usual function into a generator.

Generators are Python’s implementation of lazy evaluation. In the very first example we used a line

>>> results = s.run([0, 1, 2, 3])


The method run of a Sequence is a generator. When we call a generator, we obtain the result, but no computation really occurs, no statement from the generator’s code is executed. To actually calculate the results, the generator must be materialized. This can be done in a container (like a list or tuple) or in a cycle:

>>> for res in results:
...     print(res)


Lazy evaluation is good for:

• performance. Reading data files may be one of the longest steps in simple data analysis. Since lazy evaluation uses only one value at a time, this value can be used immediately without waiting when the reading of the whole data set is finished. This allows us to make a complete analysis in almost the same time as just to read the input data.
• low memory impact. Data is immediately used and not stored anywhere. This allows us to analyse data sets larger than the physical memory, and thus makes our program scalable.

Lazy evaluation is very easy to implement in Python using a yield keyword. Generators must be carefully distinguished from ordinary functions in Lena. If an object inside a sequence has a run method, it is assumed to be a generator. Otherwise, if the object is callable, it is assumed to be a function, which makes some simple transformation of the input value.

Generators can yield zero or multiple values. Use them to alter or reduce data flow. Use functions or callable objects for calculations that accept and return a single value.

### 3. Context¶

Lena’s goal is to cover the data analysis process from beginning to end. The final results of an analysis are tables and plots, which can be used by people.

Lena doesn’t draw anything itself, but relies on other programs. It uses a library Jinja to render text templates. There are no predefined templates or magic constants in Lena, and users have to write their own ones. An example for a one-dimensional LaTeX plot is:

% histogram_1d.tex
\documentclass{standalone}
\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=1.15}

\begin{document}
\begin{tikzpicture}
\begin{axis}[]
const plot,
]
table [col sep=comma, header=false] {\VAR{ output.filepath }};
\end{axis}
\end{tikzpicture}
\end{document}


This is a simple TikZ template except for one line: \VAR{ output.filepath }. \VAR{ var } is substituted with the actual value of var during rendering. This allows to use one template for different data, instead of creating many identical files for each plot. In that example, variable output.filepath is passed in a rendering context.

A more sophisticated example could be the following:

\BLOCK{ set var = variable if variable else '' }
\begin{tikzpicture}
\begin{axis}[
\BLOCK{ if var.latex_name }
xlabel = { $\VAR{ var.latex_name }$
\BLOCK{ if var.unit }
[$\mathrm{\VAR{ var.unit }}$]
\BLOCK{ endif }
},
\BLOCK{ endif }
]
...


If there is a variable in context, it is named var for brevity. If it has a latex_name and unit, then these values will be used to label the x axis. For example, it could become x [m] or E [keV] on the plot. If no name or unit were provided, the plot will be rendered without a label, but also without an error or a crash.

Jinja allows very rich programming possibilities. Templates can set variables, use conditional operators and cycles. Refer to Jinja documentation  for details.

To use Jinja with LaTeX, Lena slightly changed its default syntax : blocks and variables are enclosed in \BLOCK and \VAR environments respectively.

A context is a simple Python dictionary or its subclass. Flow in Lena consists of tuples of (data, context) pairs. It is usually not called dataflow, because it also has context. As it was shown earlier, context is not necessary for Lena sequences. However, it greatly simplifies plot creation and provides complementary information with the main data. To add context to the flow, simply pass it with data as in the following example:

class ReadData():

def run(self, flow):
"""Read filenames from flow and yield vectors.

If vector component could not be cast to float,
*ValueError* is raised.
"""
for filename in flow:
with open(filename, "r") as fil:
for line in fil:
vec = [float(coord)
for coord in line.split(',')]
# (data, context) pair
yield (vec, {"data": {"filename": filename}})


We read names of files from the incoming flow and yield coordinate vectors. We add file names to a nested dictionary “data” (or whatever we call it). Filename could be referred in the template as data[“filename”] or simply data.filename.

Template rendering is widely used in a well developed area of web programming, and there is little difference between rendering an HTML page or a LaTeX file, or any other text file. Even though templates are powerful, good design suggests using their full powers only when necessary. The primary task of templates is to produce plots, while any nontrivial calculations should be contained in data itself (and provided through a context).

Context allows separation of data and presentation in Lena. This is considered a good programming practice, because it makes parts of a program focus on their primary tasks and avoids code repetition.

Since all data flow is passed inside sequences of the framework, context is also essential if one needs to pass some additional data to the following elements. Different elements update the context from flow with their own context, which persists unless it is deleted or changed.

## A real analysis example¶

Now we are ready to do some real data processing. Let us read data from a file and make a histogram of x coordinates.

Note

The complete example with other files for this tutorial can be found in docs/examples/tutorial directory of the framework’s tree or online.

main.py
import os

from lena.core import Sequence, Source
from lena.math import mesh
from lena.output import ToCSV, Write, LaTeXToPDF, PDFToPNG
from lena.output import MakeFilename, RenderLaTeX
from lena.structures import Histogram

def main():
data_file = os.path.join("..", "data", "normal_3d.csv")
s = Sequence(
lambda dt: (dt, dt),
Histogram(mesh((-10, 10), 10)),
ToCSV(),
MakeFilename("x"),
Write("output"),
RenderLaTeX("histogram_1d.tex"),
Write("output"),
LaTeXToPDF(),
PDFToPNG(),
)
results = s.run([data_file])
print(list(results))

if __name__ == "__main__":
main()


If we run the script, the resulting plots and intermediate files will be written to the directory output/, and the terminal output will be similar to this:

\$ python main.py
pdflatex -halt-on-error -interaction batchmode -output-directory output output/x.tex
pdftoppm output/x.pdf output/x -png -singlefile
[(‘output/x.png’, {‘output’: {‘filetype’: ‘png’}, ‘data’: {‘filename’: ‘../data/normal_3d.csv’}, ‘histogram’: {‘ranges’: [(-10, 10)], ‘dim’: 1, ‘nbins’: }})]

During the run, the element LaTeXToPDF called pdflatex, and PDFToPNG called pdftoppm program. The commands are printed with all arguments, so that if there was an error during LaTeX rendering, you can run this command manually until the rendered file output/x.tex is fixed (and then fix the template).

The last line of the output is the data and context, which are the results of the sequence run. The elements which produce files usually yield (file path, context) pairs. In this case there is one resulting value, which has a string output/x.png as its data part.

Let us return to the script to see the sequence in more details. The sequence s runs one data file (the list could easily contain more). Since our ReadData produces a (data, context) pair, the following lambda leaves the context part unchanged, and gets the zeroth index of each incoming vector (which is the zeroth part of the (data, context) pair).

This lambda is not very readable, and we’ll see a better and more general approach in the next part of the tutorial. But it shows how the flow can be intercepted and transformed at any point within a sequence.

The resulting x components fill a Histogram, which is initialized with edges defined a mesh from -10 to 10 with 10 bins.

This histogram, after it has been fed with the complete flow, is transformed to a CSV (comma separated values) text. In order for external programs (like pdflatex) to use the resulting table, it must be written to a file.

MakeFilename adds file name to context[“output”] dictionary. context.output.filename is the file name without path and extension (the latter will be set by other elements depending on the format of data: first it is a csv table, then it may become a pdf plot, etc.) Since there is only one file expected, we can simply call it x.

Write element writes text data to the file system. It is initialized with the name of the output directory. To be written, the context of a value must have an “output” subdictionary.

After we have produced the csv table, we can render our LaTeX template histogram_1d.tex with that table and context, and convert the plot to pdf and png. As earlier, RenderLaTeX produces text, which must be written to the file system before used.

Congratulations: now you can do a complete analysis using the framework, from the beginning to the final plots. In the end of this part of the tutorial we’ll show several Lena elements which may be useful during development.

## Elements for development¶

Let us use the structure of the previous analysis and add some more elements to the sequence:

from lena.context import Context
from lena.flow import Cache, End, Print

s = Sequence(
Print(),
# Print(),
Slice(1000),
lambda val: val, # data.x
Histogram(mesh((-10, 10), 10)),
Context(),
Cache("x_hist.pkl"),
# End(),
ToCSV(),
# ...
)


Print outputs values, which pass through it in the flow. If we suspect an error or want to see exactly what is happening at a given point, we can put any number of Print elements anywhere we want. We don’t need to search for other files and add print statements there to see the input and output values.

Slice, which we met earlier when approximating pi, limits the flow to the specified number of items. If we are not sure that our analysis is already correct, we can select only a small amount of data to test that.

Context is an element, which is a subclass of dictionary, and it can be used as a context when a formatted output is needed. If a Context object is inside a sequence, it transforms the context part of the flow to its class, which is indented during output (not in one line, as a usual dict). This may help during manual analysis of many nested contexts.

Cache stores the incoming flow or loads it from file. Its initialization argument is the file name to store the flow. If the file is missing, then Cache creates that, runs the previous elements, and stores values from the flow into the file. On subsequent runs it loads the flow from file, and no previous elements are run. Cache uses pickle, which allows serialization and deserialization of most Python objects (except function’s code). If you have some lengthy calculation and want to save the results (for example, to improve plots, which follow in the sequence), you can use Cache. If you changed the algorithm before Cache, simply delete the file to refill that with the new flow.

End runs all previous elements and stops analysis here. If we enabled that in this example, Cache would be filled or read (as without the End element), but nothing would be passed to ToCSV and further. One can use End if they know for sure, that the following analysis is incomplete and will fail.

Summary

Lena encourages to split analysis into small independent elements, which are joined into sequences. This allows to substitute, add or remove any element or transform the flow at any place, which may be very useful for development. Sequences can be elements of other sequences, which allows their reuse.

Elements can be callables or generators. Simple callables can be easily added to transform each value from the flow, while generators can transform the flow, adding more values or reducing that. Generators allow lazy evaluation, which benefits memory impact and generalizes algorithms to use potentially many values instead of one.

Complete information about the analysis is provided through the context. It is the user’s responsibility to add the needed context and to write templates for plots. The user must also provide some initial context for naming files and plots, but apart from that the framework transfers and updates context itself.

We introduced two basic sequences. A Sequence can be placed before, after or inside another Sequence. A Source is similar to a Sequence, but no other sequence can precede that.

Sequences
Sequence Initialization Usage
Sequence Elements with a __call__(value) or run(flow) method (or callables) s.run(flow)
Source The first element has a __call__() method (or is callable), others form a Sequence s()

In this part of the tutorial we have learnt how to make a simple analysis of data read from a file and how to produce several plots using only one template. In the next part we’ll learn about new types of elements and sequences and how to make several analyses reading a data file only once.

Exercises

1. Ivan wants to become more familiar with generators and implements an element End. He writes this class:

class End(object):
"""Stop sequence here."""

def run(self, flow):
"""Exhaust all preceding flow and stop iteration."""
for val in flow:
pass
raise StopIteration()


and adds this element to main.py example above. When he runs the program, he gets

Traceback (most recent call last):
File “main.py”, line 46, in <module>
main()
File “main.py”, line 42, in main
results = s.run([data_file])
File “lena/core/sequence.py”, line 70, in run
flow = elem.run(flow)
File “main.py”, line 24, in run
raise StopIteration()
StopIteration

It seems that no further elements were executed, indeed. However, Ivan recalls that StopIteration inside a generator should lead to a normal exit and should not be an error. What was done wrong?

2. Svetlana wants to make sure that no statement is really executed during a generator call. Write a simple generator to check that.

3. Count counts values passing through that. In order for that not to change the data flow, it should add results to the context. What other design decisions should be considered? Write its simple implementation and check that it works as a sequence element.

4. Lev doesn’t like how the output in previous examples is organised.

“In our object-oriented days, I could use only one object to make the whole analysis”, - he says. “Histogram to CSV, Write, Render, Write again,…: if our output system remains the same, and we need to repeat that in every script, this is a code bloat”.

How to make only one element for the whole output process? What are advantages and disadvantages of these two approaches?

5. ** Remember the implementation of Sum earlier. Suppose you need to split one flow into two to make two analyses, so that you don’t have to read the flow several times or store it completely in memory.

Will this Sum allow that, why? How should it be changed? These questions will be answered in the following part of the tutorial.

The answers to the excercises are given in the end of the tutorial.

Footnotes

  This possibility may be added in the future.
  Jinja documentation: https://jinja.palletsprojects.com/
  To use Jinja to render LaTeX was proposed here and here, template syntax was taken from the original article.